The invention is in the field of transistor amplifier circuits, and relates more particularly to high-frequency amplifier circuits.
Amplifiers of this general type are frequently used in high-frequency RF amplifiers, such as those used in wireless communications apparatus. Typically, the output stage of such a high-frequency amplifier circuit uses a common emitter bipolar transistor or a common source field effect transistor, which is biased with either a voltage source or a current source at its input. The output stage is typically fed by a buffer or driver stage employing a driver transistor connected in a common collector (emitter follower) configuration for a bipolar transistor or a common drain (source follower) configuration for a field effect transistor. Buffer or driver stages of this type are typically used to increase the overall gain of the circuit and to increase the input impedance of the common emitter or common source output stage.
However, amplifier circuits employing an amplifying transistor in conjunction with a driver transistor in the configurations discussed above will typically generate a negative impedance at certain high frequencies due to circuit capacitances.
This problem has been recognized previously, and representative prior-art techniques (one of which was developed by a co-inventor herein) are illustrated in U.S. Pat. Nos. 5,424,686 and 5,828,269.
However, prior-art techniques using a resistor to compensate for the negative impedance have the drawback that the compensation resistor consumes power whenever the buffer amplifier is biased. Accordingly, it would be desirable to have a high-frequency amplifier circuit in which a resistive negative impedance cancellation effect is provided, and in which power consumption, particularly under low bias conditions, is minimized.
It is therefore an object of the present invention to provide a high-frequency amplifier circuit in which a resistive negative impedance cancellation effect is obtained in conjunction with reduced power consumption, particularly under low bias conditions.
In accordance with the invention, this object is achieved by a new high-frequency amplifier circuit including an amplifying transistor and a driver transistor, with the amplifying transistor being connected in one of a common emitter and a common source configuration and the driver transistor being connected in one of a common collector and a common drain configuration. A current-mirror bias circuit is coupled between an input terminal of the driver transistor and an output terminal of the driver transistor, and a resistor for coupling the current mirror to the input terminal of the driver transistor is provided to achieve a negative impedance cancellation effect.
In a preferred embodiment of the invention, the amplifying transistor, the driver transistor and the transistors forming the current-mirror bias circuit are all bipolar transistors, while in a further preferred embodiment all of these transistors are field effect transistors.
In yet a further preferred embodiment of the invention, the resistor for coupling the current mirror to the input terminal of the driver transistor may have a value of between about 20 and about 100 ohms.
A high-frequency amplifier circuit in accordance with the present invention offers a significant improvement in that a negative impedance cancellation effect is provided while power consumption, particularly under low-bias conditions, is reduced, in an economical and easily-implemented configuration.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.